Reserve some DDR DRAM for secure use on FVP platforms

TZC-400 is configured to set the last 16MB of DRAM1 as secure memory and
the rest of DRAM as non-secure. Non-secure software must not attempt to
access the 16MB secure area.

Device tree files (sources and binaries) have been updated to match this
configuration, removing that memory from the Linux physical memory map.
UEFI earliest commit matching this change updated in documentation.

Replaced magic numbers with #define for memory region definition in the
platform security initialization function.

Fixes ARM-software/tf-issues#149

Change-Id: Ia5d070244aae6c5288ea0e6c8e89d92859522bfe